This invention relates to an improved process for the preparation of anion exchange resins involving enhanced sulfuric acid recovery and recycle during the chloromethylation step of crosslinked poly(vinylaromatic) copolymer. In particular the present invention concerns the use of a non-agitated plug-flow quench procedure followed by recycle of recovered reaction fluids in subsequent chloromethylation reactions.
Chloromethylation of crosslinked poly(vinylaromatic) copolymer has for many years been carried out using a process where chloromethyl methyl ether (CH3OCH2Cl or CME) is prepared and reacted in situ from a reaction mixture containing formaldehyde, methanol, and chlorosulfonic acid (see Equation I). The chloromethylation of crosslinked poly(vinylaromatic) copolymer is depicted in Equation II where xe2x80x9cArxe2x80x9d represents an aromatic ring.
CH3OH+HCHO+HSO3Clxe2x86x92CH3OCH2Cl+H2SO4xe2x80x83xe2x80x83(I)
poly(Ar)+CH3OCH2Clxe2x86x92poly(ArCH2Cl)+CH3OHxe2x80x83xe2x80x83(II)
In view of toxicity and handling problems, the in situ generation and reaction of CME (such as described in EP 277,795 and U.S. Pat. No. 4,225,677) is preferable to processes that generate CME separately, for example, all-CME chloromethylations or chloromethylations where CME itself is recovered or regenerated from chloromethylation reaction mixtures and stored or added as a raw material for subsequent chloromethylation reactions (such as described in U.S. Pat. No. 4,636,554, German Patent Application DD 250,128 and U.S. Pat. No. 5,600,022).
Upon completion of the chloromethylation reaction, the reaction medium is deactivated or quenched by the addition of water or a suitable organic solvent, with mixing or agitation to dissipate heat and ensure complete deactivation. The reaction product is isolated and generally washed in an agitated fashion with water or an organic solvent as well as aqueous sodium hydroxide to remove excess sulfuric acid and degrade any unreacted CME remaining in the beads. Anion exchange resin products are then provided by means of conventional amination reactions. A major disadvantage of the use of CSA as a source of HCl or as a H2SO4 reaction medium in chloromethylation reactions is the generation of large volumes of waste acid that require treatment and disposal for environmental reasons.
The problem addressed by the present invention is to overcome the deficiencies of prior methods used to prepare anion exchange resins based on sulfuric acid chloromethylations by minimizing usage and subsequent waste disposal requirements of sulfuric acid, while still relying on the conventional sulfuric acid chloromethylation reactions to provide the desired anion exchange resins without any reduction in physical stability characteristics.
The present invention provides an improved method for preparing anion exchange resins in a multi-batch process wherein a crosslinked poly(vinylaromatic) copolymer is chloromethylated in the presence of sulfuric acid to provide a mixture of a chloromethylated intermediate and sulfuric acid, followed by quenching the chloromethylated intermediate with quenching solution and finally functionalizing the chloromethylated intermediate with an aminating agent, the improvement comprising (a) quenching the chloromethylated intermediate by a non-agitated plug-flow downward addition of a quench solution to the mixture of chloromethylated intermediate and sulfuric acid to obtain a mother liquor effluent comprising 30 to 90% sulfuric acid; (b) isolating the mother liquor effluent for reuse in subsequent chloromethylation reactions; and (c) chloromethylating additional crosslinked poly(vinylaromatic) copolymer in the presence of sulfuric acid and a portion of mother liquor effluent comprising 30 to 90% sulfuric acid obtained from a prior chloromethylation-quench reaction sequence.
In a further specific embodiment, the present invention provides a method for preparing anion exchange resins in a multi-batch process, comprising (i) in a first chloromethylation reaction, the steps of (a) chloromethylating a first crosslinked poly(vinylaromatic) copolymer in the presence of sulfuric acid to provide a mixture of a first chloromethylated intermediate and sulfuric acid; (b) quenching the first chloromethylated intermediate to provide a quenched first chloromethylated intermediate by a non-agitated plug-flow downward addition of a quench solution to the mixture of first chloromethylated intermediate and sulfuric acid to obtain a first mother liquor effluent comprising 30 to 90% sulfuric acid; and (c) isolating the first mother liquor effluent for reuse in one or more subsequent chloromethylation reactions; and (ii) in a second chloromethylation reaction, the steps of (axe2x80x2) chloromethylating a second crosslinked poly(vinylaromatic) copolymer in the presence of sulfuric acid and a portion of mother liquor effluent obtained from a prior chloromethylation-quench reaction sequence to provide a mixture of a second chloromethylated intermediate and sulfuric acid; (bxe2x80x2) quenching the second chloromethylated intermediate to provide a quenched second chloromethylated intermediate by a non-agitated plug-flow downward addition of a quench solution to the mixture of second chloromethylated intermediate and sulfuric acid to obtain a second mother liquor effluent comprising 30 to 90% sulfuric acid; and (cxe2x80x2) isolating the second mother liquor effluent for reuse in one or more subsequent chloromethylation reactions. In another aspect, the present invention provides the latter method wherein a portion of the first or second mother liquor effluent is used in one or more subsequent chloromethylation reactions that are repeated as described in part (ii), steps (axe2x80x2) through (cxe2x80x2).